The present invention relates to fluoroelastomers curable with peroxides and having improved mechanical and elastomeric properties combined with improved processability and extrudability.
Various kinds of fluoroelastomers, are widely used in technical applications where products are required to have elastomeric properties combined with high thermochemical stability. A detailed description of such products is presented in xe2x80x9cUllmann""s Encyclopedia of Industrial Chemistryxe2x80x9d, Volume A-11, pages 417-429 (1988, VCH Verlagsgesellschaft).
Fluoroelastomer curing can be carried out by ionic- and peroxide-based methods. In the former case, fluoroelastomer curing agents such as polyhydroxylated compounds are combined with accelerating agents such as tetraalkylammonium, tetraalkylphosphonium phosphoranamine or salts thereof. For peroxide-based curing, the polymer must contain reactive groups capable of forming radicals in the presence of peroxides. Monomers containing reactive groups such as iodine and/or bromine can be introduced into the polymeric skeleton as described in U.S. Pat. Nos. 4,035,565, 4,475,165 and EP 199,138. Chain transfer agents containing iodine and/or bromine, which generate iodinated and/or brominated end groups, can also be used in the polymerization-phase (U.S. Pat. Nos. 4,243,770 and 5,173,553).
A drawback of the compounds used for peroxidic curing is their difficult processability. Fluoroelastomers cured by peroxides compared with those cured by ionic compounds are reduced in their elastomeric properties, e.g., high compression set values and moldability, which results in decreased product yield.
There has been a longfelt need for peroxide-curable fluoroelastomers having improved mechanical and elastomeric properties combined with an improved extrudability.
The inventor has surprisingly and unexpectedly found new peroxide curable fluoroelastomers having improved mechanical and elastomeric properties combined with improved processability, in particular improved extrudability.
An object of the present invention is a peroxide-curable fluoroelastomer having iodine atoms at the terminal ends comprising monomeric unites formed by a triazine iodinated derivative of formula: 
wherein CYxe2x80x22, CXxe2x80x22, Xxe2x80x22C and Yxe2x80x22C represent carbon atoms bound to two Yxe2x80x2 and Xxe2x80x2 substituents as defined below:
Yxe2x80x2 can independently be H, Cl, F, or CH3;
mxe2x80x2 and txe2x80x2 are 0 or 1, where mxe2x80x2+txe2x80x2=0 or 1, preferably mxe2x80x2+txe2x80x2=0;
pxe2x80x2 is 0 or 1, and is equal to 1 when txe2x80x2=1;
Xxe2x80x2 can independently be H, Cl, F, alkyl or perfluoroalkyl C1-C3, preferably F;
nxe2x80x2 is an integer in the range of 2-20, preferably 4-12, more preferably 4-8.
The preferred compounds of formula (I) are those wherein mxe2x80x2=txe2x80x2=pxe2x80x2=0; nxe2x80x2 is between 4 and 8; and Xxe2x80x2=F.
The concentration of the triazine iodinated derivatives in the polymer chain is generally in the range of 0.01-1.0 moles, preferably 0.03-0.5 moles, more preferably 0.05-0.2 moles per 100 moles of the other monomeric units forming the polymer.
The presence of a triazine iodinated derivative of formula (I) results in polymers having a very narrow molecular weight distribution as determined by GPC. The inventor has found that a narrow molecular weight distribution for the polymer is a contributing factor to the improved extrudability of the product.
The fluorelastomeric polymers described hereunder, besides having improved processability and extrudability, show a combination of improved mechanical and elastomeric properties, in particular a lower compression set point.
The fluoroelastomer base structure is selected from at least one of the class of copolymers comprising two or more monomers comprising:
(1) VDF-based copolymers, wherein VDF is copolymerized with at least one comonomer selected from the group consisting of:
perfluoroolefins C2-C8 such as tetrafluoroethylene (TFE) or hexafluoropropene (HFP), chloro-, bromo- or iodo-fluoroolefins C2-C8 such as chlorotrifluoroethylene (CTFE) and bromotrifluoroethylene, (per)fluoroalkylvinylethers (PAVE) CF2xe2x95x90CFORf, wherein Rf is a (per)-fluoroalkyl C1-C6 such as trifluoromethyl, bromodifluoromethyl, or
pentafluoropropyl, perfluoro-oxyalkylvinylethers CF2xe2x95x90CFOX, wherein X is a perfluorooxyalkyl C1-C12 having one or more ether groups such as perfluoro-2-propoxy-propyl, and non fluorinated olefins (Ol) C2-C8 such as ethylene and propylene; and
(2) TFE based copolymers, wherein TFE is copolymerized with at least one of a comonomer selected from the group consisting of:
(per) fluoroalkylvinylethers (PAVE) CF2xe2x95x90CFORf, wherein Rf is as above defined, perfluorooxyalkylvinylethers CF2xe2x95x90CFOX, wherein X is as above defined, fluoroolefins C2-C8 containing hydrogen, chlorine, bromine or iodine atoms and non-fluorinated olefins (Ol) C2-C8.
Preferably the fluoroelastomers contain perfluorinated monomers, and more preferably, the base structure of the fluoroelastomers are selected from the copolymers of class (2), wherein TFE is polymerized with one or more perfluorinated comonomers as above indicated.
The preferred compositions of the monomers forming the fluoroelastomer base structure comprise
(a) VDF 45-85%, HFP 15-45%, TFE 0-30%;
(b) VDF 50-80%, PAVE 5-50%, TFE 0-20%;
(c) VDF 20-30%, Ol 10-30%, HFP e/o PAVE 18-27%, TFE 10-30%;
(d) TFE 50-80%, PAVE 20-50%;
(e) TFE 45-65%, Ol 20-55%, VDF 0-30%;
(f) TFE 32-60%, Ol 10-40%, PAVE 20-40%; and
(g) TFE 33-75%, PAVE 15-45%, VDF 5-30%.
Specific compositions particularly preferred are the following:
d) TFE 50-80%, PAVE 20-50%;
g) TFE 33-75%, PAVE 15-45%, VDF 5-30%.
The fluoroelastomers optionally comprise monomeric units derived from a bis-olefin of formula: 
wherein
R1, R2, R3, R4, R5 and R6 are equal to or different from each other, and are H or C1-C5 alkyl;
Z is an alkylenic or cycloalkylenic C1-C18 radical, linear or branched, optionally containing oxygen atoms. Preferably z is at least partially fluorinated or a (per)fluoropolyoxyalkylene radical.
In formula (II), Z is preferably a perfluoroalkylene C4-C12 radical, while R1, R2, R3, R4, R5 and R6 are preferably hydrogen.
When Z is a (per) fluoropolyoxyalkylene radical, preferably the formula is
xe2x80x94(Q)pxe2x80x94CF2Oxe2x80x94(CF2CF2O)m(CF2O)nxe2x80x94CF2xe2x80x94(Q)pxe2x80x94xe2x80x83xe2x80x83(III),
wherein
Q is an alkylene or oxyalkylene C1-C10 radical;
p is 0 or 1;
m and n are integers such that the m/n ratio is in the range of 0.2-5 and the molecular weight of the (per)fluoro-polyoxyalkylene radical is in the range of 500-10,000, preferably 1,000-4,000.
Preferably Q is selected from the group consisting of xe2x80x94CH2OCH2xe2x80x94; and xe2x80x94CH2O(CH2CH2O)sCH2xe2x80x94, where s is an integer from 1 to 3.
Bis-olefins of formula (II), wherein Z is an alkylene or cycloalkylene radical can be prepared according to the methods of I. L. Knunyants et al Izv. Akad. Nauk. SSSR, Ser. Khim., 1964(2), 384-6), while the bisolefins containing (per)fluoropolyoxyalkylene sequences are described in U.S. Pat. No. 3,810,874.
The concentration of the bis-olefins in the polymeric chain is generally in the range of 0.01-1.0 moles, preferably 0.03-0.5 moles, more preferably 0.05-0.2 moles per 100 moles of the other above mentioned monomeric unites forming the polymer base structure.
The fluoroelastomers of the present invention in addition to having iodinated end groups derived from the triazine derivative, can optionally contain iodine and/or bromine atoms. Iodine and/or bromine atoms can be introduced to the reaction mixture by the addition of brominated and/or iodinated cure-site comonomers such as bromo- and/or iodo-olefins having from 2 to 10 carbon atoms as described in U.S. Pat. Nos. 4,035,565 and 4,694,045, or iodo- and/or bromo-fluoroalkylvinylethers as described in U.S. Pat. Nos. 4,475,165, 5,564,662 and EP 199,138). The concentration for the cure-site comonomers in the final product is generally in the range of 0.05-2 moles per 100 moles of the other base monomeric units.
A process for preparing the fluoroelastomers is another object of the present invention. The process can be carried out by copolymerization of the monomers in aqueous emulsion in the presence of radical initiators such as alkaline or ammonium persulphates, perphosphates, perborates or percarbonates, or optionally in association with ferrous, cuprous or silver salts, or other easily oxidable metals. The reaction medium can also contain various surfactants among which are the fluorinated surfactants of formula Rf2xe2x80x94X2xe2x80x94M+, wherein Rf2 is a (per)fluoroalkyl chain C5-C16 or a (per)fluoropolyoxyalkylene chain, X2xe2x80x94 is xe2x80x94COOxe2x88x92 or xe2x80x94SO3xe2x88x92, and M+ is selected from the group consisting of H+, NH4+, and alkaline metal ion.
More preferably, ammonium perfluorooctanoate and (per)fluoropolyykylens ending with one or more carboxylic groups are used as surfactants.
The amount of triazine derivative (I) added to the reaction mixture can be adjusted depending on the desired final concentration for the final product.
Upon completion of the polymerization reaction, the fluoroelastomer can be isolated by coagulation (adding electrolytes) or by cooling.
Alternatively, polymerization can be carried out in bulk or in a suspension comprising an organic liquid in the presence of a radical initiator.
The polymerization is generally carried out at temperatures in the range of 25-150xc2x0 C. under pressure up to 10 Mpa.
The preparation of the fluoroelastomers of the present invention is preferably carried out in aqueous emulsion in the presence of a perfluoropolyoxyalkylene emulsion, dispersion or microemulsion according to U.S. Pat. Nos. 4,478,717 and 4,864,006.
Peroxide curing of fluoroelastomers is carried out by addition of a peroxide capable of generating radicals upon heating. Among the most commonly used peroxides are dialkylperoxides such as di-terbutyl-peroxide and 2,5-dimethyl-2,5-di(terbutylperoxy)hexane; dicumyl peroxide; dibenzoyl peroxide; diterbutyl perbenzoate; and di[1,3-dimethyl-3-(terbutylperoxy)butyl-]carbonate. Other peroxides are described in EP 136,596 and EP 410,351.
The fluoroelastomer compound may contain additives such as:
(a) curing coagents in amounts in the range of 0.5-10%, preferably 1-7%, by weight with respect to the polymer. Curing coagents commonly used are triallyl-cyanurate, triallyl-isocyanurate (TAIC), tris(diallylamine)-s-triazine, triallylphosphite, N,N-diallyl-acrylamide, N,N,Nxe2x80x2,Nxe2x80x2-tetraallyl-malonamide, trivinyl-isocyanurate, 2,4,6-trivinyl-methyltrisiloxane, N,Nxe2x80x2bisallylbicyclo-oct-7-ene-disuccinimide (BOSA), bis olefin of formula (I), and triazines having formula 
wherein CXxe2x80x22, Xxe2x80x2 and nxe2x80x2 are as defined in formula (I).
Preferably in formula (IV), nxe2x80x2 is from 4 to 8, and TAIC is particularly preferred;
(b) a metal compound in amounts in the range of 1-15% by weight, preferably 2-10%, with respect to the polymer. Metals are selected from the group of divalent metal oxides or hydroxides such as Mg, Zn, Ca, Pb, and optionally associated with a monovalent or bivalent metal salt of an organic or inorganic weak acid such as Ba, Na, K, Pb, Ca stearates, benzoates, carbonates, oxalates or phosphates;
(c) mineral fillers such as carbon black, barium sulphate, PTFE with a particle diameter lower than 300 nm, preferably lower than 100 nm, more preferably from 30 to 70 nm. PTFE of 30-70 nm in size is preferable. Alternatively, polytetrafluoroethylene (PTFE) or TFE plastomeric polymers such as TFE polymers modified with amounts from 0.01% to 10% by moles, preferably from 0.01 to 4% by moles of a vinylether, preferably perfluoromethylvinylether, perfluoroethylvinyl ether, perfluoropropylvinylether can be used. Preferably, plastomeric polymers are TFE modified with MVE; and
(d) other additives such as thickeners, pigments, antioxidants, stabilizers and the like.
A process for curing fluoroelastomers may also encompass a mixed system where both ionic and peroxidic compounds are used as curing agents as described in EP 136,596.
The inventive fluoroelastomers show improved extrudability as well as processability, thus allowing one to obtain a higher production yield and a reduction in the generation of processing waste materials.
With the fluoroelastomers of the present invention, it is possible to prepare manufactured articles such as fuel hoses, O-rings, shaft seals, and gaskets, preferably fuel hoses having improved compression set and a very good extruded product (Garvey rating).
The triazine derivatives of formula (I) can be prepared by the following process as described in U.S. Pat. No. 5,910,587:
a) reaction of a compound of formula:
Ixe2x80x94(CXxe2x80x22xe2x80x94CYxe2x80x22O)mxe2x80x2xe2x80x94(CXxe2x80x22)nxe2x80x2+1+mxe2x80x2xe2x80x94I, wherein CYxe2x80x22, CXxe2x80x22, Xxe2x80x2, Yxe2x80x2, mxe2x80x2 and nxe2x80x2 are as defined in formula (I), in the presence of an oxide or a transition metal salt (e.g., HgSO4), and of oleum containing an amount of SO3 comprising between 5 to 60% by weight, preferably between 10 to 40% by weight, to obtain an omega iodoacylfluoride,
b) reacting the omega iodoacylfluoride with ammonia in an inert solvent such as methylene chloride, ethylic ether, perfluoroheptane or the like, for between 10 minutes to 2 hours to obtain the corresponding amide,
c) reacting the amide with a dehydrating agent (e.g., P2O5) to obtain the corresponding nitrile,
d) reacting the nitrile with ammonia at a temperature between xe2x88x9210 and xe2x88x92100xc2x0 C. to obtain the corresponding amidine,
e) condensing the amide at a temperature between 120 and 170xc2x0 C. to obtain the triazine (I).
The reaction temperature can vary widely, more preferably between 80 to 150xc2x0 C.
The present invention is better illustrated by the following working examples which define the purpose of the invention but are non-limiting as to the scope thereof.